Followers.

A requirement of the Building Regulations is that walls should adequately resist the passage of moisture to the inside of the building. Moisture includes water vapour and liquid water. Moisture may penetrate a wall by absorption of water from the ground that is in contact with foundation walls or through rain falling on the wall.
To prevent water, which is absorbed from the ground by foundation walls, rising in a wall to a level where it might affect the inside of a building it is necessary to form a continuous, horizontal layer of some impermeable material in the wall. This impermeable layer, the damp- proof course, is built in, some 150 mm above ground level, to all foundation walls in contact with the ground and is joined to the damp-proof membrane in solid ground floors.

The ability of a wall to resist the passage of water to its inside face depends on its exposure to wind driven rain and the construction of the wall. The exposure of a wall is determined by its location and the extent to which it is protected by surrounding higher ground, or sheltered by surrounding buildings or trees, from rain driven by the prevailing winds. In Great Britain the prevailing, warm westerly winds from the Atlantic Ocean cause more severe exposure to driving rain along the west coast of the country than do the cooler easterly winds on the east coast.

British Standard 5628: Part 3 defiuies five categories of exposure as:
very severe; moderate/severe; sheltered/moderate; sheltered; and very sheltered. A map of Great Britain, published by the Building Research Establishment, shows contours of the variations of exposure across the country. The contour lines, indicating the areas of the categories of exposure, are determined from an analysis of the most severe likely spells of wind driven rain, occurring on average every 3 years, plotted on a 10 km grid. The analysis is based on the ‘worst case’ for each geographical area, where a wall faces open country and the prevailing wind, such as a gable end wall on the edge of a suburban site facing the prevailing wind or a wall of a tall building on an urban site rising above the surrounding buildings and facing the prevailing wind.
Where a wall is sheltered from the prevailing winds by adjacent high ground or surrounding buildings or trees the exposure can be reduced by one category in sheltered areas of the country and two in very severe exposure areas of the country. The small-scale and large- scale maps showing categories of exposure to driving rain provide an overall picture of the likely severity of exposure over the country. To estimate the likely severity of exposure to driving rain, of the walls of a building on a particular site, it is wise to take account of the categories of exposure shown on the maps, make due allowance for the overlap of categories around contour lines and obtain local knowledge of conditions from adjacent buildings and make allowance for shelter from high ground, trees and surrounding buildings.

The behaviour of a wall in excluding wind and rain will depend on the nature of the materials used in the construction of the wall and how they are put together. A wall of facing bricks laid in mortar will absorb an appreciable amount of the rain driven on to it so that the wall must be designed so that the rain is not absorbed to the inside face of the wall. This may be effected by making the wall of sufficient thickness, by applying an external facing of say rendering or slate hanging, or by building the wall as a cavity wall of two skins or leaves with a separating cavity.

A curtain wall of glass on the other hand will not absorb water through the impermeable sheets of glass so that driving rain will pour down the face of the glass and penetrate the joints between the sheets of glass and the supporting frame of metal or wood, so that close attention has to be made to the design of these joints that at once have to be sufficiently resilient to accommodate thermal movement and at the same time compact enough to exclude wind and rain.

It is generally accepted practice today to construct walls of brick, stone or blocks as a cavity wall with an outer and inner leaf or skin separated by a cavity of at least 50 mm. The outer leaf will either be sufficiently thick to exclude rain or be protected by an outer skin of rendering or cladding of slate or tile and the inner leaf will be constructed of brick or block to support the weight of floors and roofs with either the inner leaf providing insulation against transfer of heat or the cavity filled with some thermal insulating material.